Magnetic Random Access Memory (MRAM) is a non-volatile computer memory technology based on magnetoresistance. One type of MRAM cell is a spin torque transfer MRAM (STT-MRAM) cell, such as that illustrated in FIG. 1. A conventional STT-MRAM cell includes a magnetic cell core 100 supported by a substrate 102. The magnetic cell core 100 includes at least two magnetic regions, for example, a “fixed region” 130 and a “free region 170,” with a non-magnetic region 160 in between. One or more lower intermediary regions 120 and one or more upper intermediary regions 180 may be disposed under and over, respectively, the magnetic regions (e.g., the fixed region 130 and the free region 170) of the magnetic cell core 100 structure.
An STT-MRAM cell configured to exhibit perpendicular magnetic anisotropy (“PMA”) includes the fixed region 130 that has a fixed, vertical magnetic orientation and includes the free region 170 that has a vertical magnetic orientation that may be switched, during operation of the cell, between a “parallel” configuration (FIG. 1) and an “anti-parallel” configuration (FIG. 2). In the parallel configuration (FIG. 1), a magnetic orientation 171 of the free region 170 is directed essentially in the same direction (e.g., north or south) as a magnetic orientation 131 of the fixed region 130, giving a lower electrical resistance across the magnetoresistive elements, i.e., the fixed region 130 and free region 170. This state of relatively low electrical resistance may be defined as a “0” state of the MRAM cell. In the anti-parallel configuration (FIG. 2), a magnetic orientation 172 of the free region 170 is directed essentially in the opposite direction (e.g., north or south) of the magnetic orientation 131 of the fixed region 130, giving a higher electrical resistance across the magnetoresistive elements, i.e., the fixed region 130 and free region 170. This state of relatively high electrical resistance may be defined as a “1” state of the MRAM cell.
Switching of the magnetic orientation 171, 172 of the free region 170 and the resulting high or low resistance states across the magnetoresistive elements enables the write and read operations of the typical MRAM cell. In operation, a programming current may be caused to flow through an access transistor and the magnetic cell core 100. The fixed region 130 within the magnetic cell core 100 polarizes the electron spin of the programming current, and torque is created as the spin-polarized current passes through the magnetic cell core 100. The spin-polarized electron current interacts with the free region 170 by exerting a torque on the free region 170. When the torque of the spin-polarized electron current passing through the magnetic cell core 100 is greater than a critical switching current density (Jc) of the free region 130, the torque exerted by the spin-polarized electron current is sufficient to switch the direction of the magnetization, i.e., between magnetic orientation 171 and magnetic orientation 172, of the free region 170. Thus, the programming current can be used to cause the magnetic orientation 171, 172 of the free region 170 to be aligned either parallel to (FIG. 1) or anti-parallel to (FIG. 2) the magnetic orientation 131 of the fixed region 130.
Ideally, the amount of programming current required to switch the free region 170 from the parallel configuration (FIG. 1) to the anti-parallel configuration (FIG. 2) is essentially the same amount of programming current required to switch from the anti-parallel configuration (FIG. 2) to the parallel configuration (FIG. 1). Such equal programming current for switching is referred to herein as “symmetric switching.”
Though symmetric switching may be ideal, in conventional magnetic cell cores 100, one or more magnetic regions, because of their magnetic natures, may emit a magnetic dipole field, which may interfere with switching in the free region 170. For example, a magnetic dipole field 132 may be emitted by the fixed region 130, as illustrated in FIGS. 1 and 2. (Notably, though the magnetic dipole field 132 is illustrated as passing between essentially the entirety of an upper surface and a lower surface of the fixed region 130, in actuality, the fixed region 130 may have a height substantially smaller than the width of the fixed region 130, such that the magnetic dipole field 132 may be emitted from upper and lower surfaces essentially proximate only to sidewalls of the fixed region 130.) When the free region 170 is in one configuration, e.g., the parallel configuration (FIG. 1), the magnetic orientation 171 of the free region 170 may be in at least partial parallel alignment with the magnetic dipole field 132 from the fixed region 130; however, when the free region 170 is in the other configuration, e.g., the anti-parallel configuration (FIG. 2), the magnetic orientation 172 of the free region 170 may be in at least partial anti-parallel alignment with the magnetic dipole field 132. As illustrated in FIGS. 1 and 2, then, the magnetic dipole field 132 may be emitted from an upper surface of the fixed region 130 and pass through a portion of the free region 170 before arcing to enter a lower surface of the fixed region 130. When the free region 170 is in the parallel configuration (FIG. 1), both the magnetic dipole field 132 from the fixed region 130 and the magnetic orientation 171 of the free region 170 may be directed in essentially the same direction (e.g., upwards and upwards, respectively). However, when the free region 170 is in the anti-parallel configuration (FIG. 2), the magnetic dipole field 132 from the fixed region 130 and the magnetic orientation 172 of the free region 170 may be directed in essentially opposite directions (e.g., upwards and downwards, respectively). Hence, the free region 170 may have a higher affinity for being in the parallel configuration (FIG. 1) than in the anti-parallel configuration (FIG. 2) such that more programming current may be needed to switch the free region 170 to the anti-parallel configuration (FIG. 2) from the parallel configuration (FIG. 1) than is needed to switch the free region 170 from the anti-parallel configuration (FIG. 2) to the parallel configuration (FIG. 1). The presence of the magnetic dipole field 132 emitted from the fixed region 130 may, therefore, impair the ability to symmetrically switch the magnetic orientation 171, 172, of the free region 170 during operation of the MRAM cell.
Efforts have been made to eliminate the negative effects on switching due to interference from a stray magnetic dipole field 132. These efforts include, for example, attempts to neutralize the magnetic dipole field 132 by balancing magnetic orientations within the magnetic region, e.g., the fixed region 130. For example, FIG. 3 illustrates a conventional fixed region 330 including magnetic material 334 separated by conductive material 336. A coupler material 338 couples a lower region and an upper region of the fixed region 330. The conductive material 336, disposed between the magnetic material 334, causes the magnetic material 334 to exhibit a perpendicular anisotropy, i.e., vertical magnetic orientations 331, 333, while the coupler material 338 is formulated and positioned to provide anti-parallel coupling of adjacent magnetic material. Thus, the fixed region 330 is configured as a synthetic antiferromagnet (SAF) with the upper region and the lower region of the fixed region 330 coupled via a single intervening coupler material 338. The goal is that a magnetic dipole field emitted by the upper region will be effectively cancelled by a magnetic dipole field emitted by the lower region due to the opposite directions of the magnetic orientations 331, 333. However, the free region of the cell will be disposed closer to one of the upper and lower regions of the fixed region 330 such that the free region will experience the magnetic dipole field emitted by the more proximal of the upper and lower regions more strongly than the free region will experience the other magnetic dipole field. Thus, balancing the magnetic orientations of the upper and lower regions may not effectively cancel a magnetic dipole field experienced by the free region of the cell. Hence, designing a cell core structure that achieves symmetrical switching of the free region has been a challenge.